1. Field of the Invention
The present invention relates generally to methods for forming apertures at least partially through substrate layers within integrated circuits. More particularly, the present invention relates to a method for forming a narrow aperture width patterned positive photoresist layer which may be employed as a positive photoresist etch mask layer in forming a narrow aperture width contact or interconnection via through an insulator layer within an integrated circuit.
2. Description of the Related Art
Integrated circuits are formed from semiconductor substrate within and upon whose surfaces are formed resistors, transistors, diodes and other electrical circuit elements. The electrical circuit elements are connected internally and externally to the semiconductor substrate upon which they are formed through patterned conductor layers which are separated by insulator layers.
As integrated circuit technology has advanced, and the dimensions of electrical circuit elements within integrated circuits have decreased, it has become increasingly important within those advanced integrated circuits to form through insulator layers within those advanced integrated circuits narrow aperture width contact and interconnection vias of commensurately smaller dimensions to make electrical circuit connections to the electrical circuit elements of decreased dimensions. Typically, narrow aperture width contact and interconnection vias through insulator layers within advanced integrated circuits are formed to the most narrow of narrow aperture dimensions and tolerances employed within advanced integrated circuits. Since narrow aperture width contact and interconnection vias through insulator layers within advanced integrated circuits are formed to the most narrow aperture dimensions and tolerances, it is common in the art that those narrow aperture width contact and interconnection vias are of ten formed only with difficulty. The difficulty in forming narrow aperture width contact and interconnection vias of the most narrow aperture dimensions and tolerances within advanced integrated circuits typically derives from corresponding difficulties in forming a narrow aperture width patterned positive photoresist etch mask layer upon the surface of the insulator layer through which the narrow aperture width contact or interconnection via is desired to be formed.
Among the difficulties included in forming a narrow aperture width patterned positive photoresist etch mask layer upon an insulator layer where the narrow aperture width patterned positive photoresist etch mask layer is employed in forming a narrow aperture width contact or interconnection via through the insulator layer are included: (1) the additional photoexposure energy typically needed to fully photoexpose the portion of a blanket positive photoresist layer which defines the aperture width of the narrow aperture width patterned positive photoresist etch mask layer, particularly when the aperture width is exceedingly narrow (ie: less than about 0.4 microns); and (2) and the inherently reduced depth of focus typically encountered when forming from the blanket positive photoresist layer the narrow aperture width patterned positive photoresist etch mask layer, also particularly when the aperture width is exceedingly narrow (ie: less than about 0.4 microns).
The difficulty relating to the additional photoexposure energy is typically addressed by extending the photoexposure time when forming a narrow aperture width patterned positive photoresist etch mask layer from a blanket positive photoresist layer. Unfortunately, extended photoexposure times often significantly decrease manufacturing efficiency, throughput and economy. The difficulty relating to the inherently reduced depth of focus is often, although not exclusively, compensated by decreasing the thickness of the blanket positive photoresist layer employed in forming the narrow aperture width patterned positive photoresist etch mask layer. Unfortunately, substantial reductions in thickness of the blanket positive photoresist layer may yield a narrow aperture width patterned positive photoresist etch mask layer of insufficient thickness to pattern with adequate uniformity, while employing the narrow aperture width patterned positive photoresist etch mask layer, a contact or interconnection via through an insulator layer formed beneath the narrow aperture width patterned positive photoresist etch mask layer. Thus, the foregoing photoexposure energy and depth of focus difficulties present genuine problems in forming from blanket positive photoresist layers narrow aperture width patterned positive photoresist etch mask layers for use in forming narrow apertures, such as narrow contact and interconnection vias, through substrate layers, such as insulator layers, which are formed beneath those narrow aperture width patterned positive photoresist etch mask layers.
Various methods have been disclosed in the art of integrated circuit fabrication for forming patterned photoresist layers from blanket photoresist layers. For example, Koh, in U.S. Pat. No. 5,286,608 discloses a method which employs a conductive thermally oxidized titanium oxide layer as an anti-reflective coating (ARC) layer when forming a patterned photoresist layer from a blanket photoresist layer within an integrated circuit. In addition, Sardella, in U.S. Pat. No. 5,310,622, discloses a method employing: (1) a first photoresist layer of lower optical transmittance formed upon a reflective layer in an integrated circuit; and (2) a second photoresist layer of higher optical transmittance formed upon the first photoresist layer. Through the method, the bilayer of the second photoresist layer and the first photoresist layer may be patterned with enhanced dimensional uniformity due to attentuated reflectivity from the reflective layer.
It is thus desirable in the art of integrated circuit fabrication to provide a method for forming from a blanket positive photoresist layer a narrow aperture width patterned positive photoresist etch mask layer, where the narrow aperture width patterned positive photoresist etch mask layer may subsequently be employed in forming within or through a substrate layer, such as an insulator layer, formed beneath the narrow aperture width patterned positive photoresist etch mask layer a narrow aperture, such as a narrow contact or interconnection via. Particularly desirable are methods through which there may be formed from the blanket positive photoresist layer the narrow aperture width patterned positive photoresist etch mask layer while simultaneously: (1) limiting the photoexposure time in forming the narrow aperture width patterned positive photoresist etch mask layer from the blanket positive photoresist layer; and (2) compensating for the depth of focus limitation typically encountered in forming the narrow aperture width patterned positive photoresist etch mask layer from the blanket positive photoresist layer, particularly when forming exceedingly narrow apertures (ie: less than about 0.4 microns) within the narrow aperture width patterned positive photoresist etch mask layer. It is towards providing a method in accord with these goals that the present invention is directed.